The invention relates to a communication bus system in which a plurality of stations is connected by signal transmission lines to enable communication between the stations using a bus like the USB bus.
A USB system has connectors, which the user may use to connect and disconnect stations at will, even when the system is running. The system detects whether a station has been connected to a connector and, if so, it logically incorporates the connected station into the system, so that communication with the station becomes possible. The system also detects when a station has been disconnected and; if so, logically disincorporates the station from the system, so that no more communication with the station is performed or expected. This automatic incorporation and disincorporation is an important feature to make the system easy to use for non-specialist consumers.
In a USB system one station, for example a personal computer, is at the root of the bus structure. This station is the bus master. This station has one or more connectors, which can be used to connect the root to xe2x80x9cdownstreamxe2x80x9d stations, each via its own cable connected to a connector. In the USB system the downstream stations in turn may have connectors that may be used to connect them to further downstream stations and so on in a tree structure. The interconnected stations, which are thus directly or indirectly coupled to the root, together form the bus system.
It is not necessary that all connectors are permanently connected to downstream stations. The user of the system may connect or disconnect a downstream station or leave the connector unused as desired. The system detects whether or not a downstream station is connected to or disconnected from the connector and operates accordingly: no messages commands etc. are transmitted via connectors to which no downstream station is connected. In the USB system detection of the presence of a station is performed by means of current drawn through resistors. Each downstream station contains a resistor connected to the cable that connects the downstream station to the connector via which it is connected to the bus system. When a downstream station is connected to the bus, its resistor draws current via the cable. This current causes a change in the voltage on the cable and this change is detected by another xe2x80x9cupstreamxe2x80x9d station to whose connector the downstream station is connected.
Normally, detection of disconnection takes a period of xe2x80x9csilencexe2x80x9d in the communication on the bus. In the case of the USB bus, a time-slot is reserved for this purpose, during which no transmission is possible. This may cause a delay in disconnection and reduced transmission capacity. Furthermore it is desirable to reduce the voltage swing on the cables, both for reasons of increasing the speed and for reducing the power consumption of the bus system. However, a reduction of the voltage swing makes the detection of the voltage change due to connection of a station harder to detect.
Amongst others, it is an object of the invention to reduce the time that the bus needs to reserve to detect connection and/or disconnection.
An apparatus according to the invention is set forth in claim 1. According to the invention wave reflection is used to detect the presence or absence of stations coupled to the bus system. The time dependent voltages and currents in an electrical transmission line for example can be described by two vectors, each representing the phase and amplitude of a respective one of two traveling waves that travel through the transmission line in mutually opposite directions. At an end of the transmission line, where an impedance is connected to the transmission line, the ratio between the two wave vectors is equal to the reflection coefficient associated with the impedance. The reflection coefficient is zero when the impedance is equal to the transmission line impedance of the transmission line. As a result there will be a wave traveling in only one direction when an impedance equal to the characteristic impedance of the transmission line is connected to the transmission line.
According to the invention this effect is used to control the incorporation and especially disincorporation of downstream stations into and from the system. Stations are designed to apply the characteristic impedance of the transmission line to the transmission line. An upstream station in the system located at the other end of the transmission line transmits a wave signal via the transmission line and spits off the returning wave (if any) from the transmitted wave. If during transmission the amplitude of the returning wave is not substantially zero or substantially increases from zero, it is concluded that the transmission line is no longer terminated and therefore that a downstream station is disconnected from the transmission line. Thereupon the upstream station takes the appropriate actions to remove the downstream station logically from the system. Similarly, to enter a station logically into the system, if the amplitude of the returning wave is substantially zero, it is concluded that the transmission line is terminated and therefore that a downstream station is connected to the transmission line. Thereupon the upstream station takes the appropriate actions to incorporate the downstream station into the system.